In the lubricating oils used at low temperature, in general, pour-point depressant and viscosity index improver are blended to improve the low-temperature properties. For example, as about engine oils, evaluation of cold cranking limit in according with CCS viscosity (ASTM D 5293) and of low-temperature pumping limit in according with MRV viscosity (ASTM D 4684), and so on are carried out; while, as about gear oils and transmission oils, evaluation of cold flow in accordance with BF viscosity (ASTM D 2983), and so on are carried out.
On the other hand, as about lubricating oils for tractor, the one particular lubricating oil is not only required to have ability of lubricating transmission, gears, bearings, hydraulic systems, power steering, wet-type brake, but also required to have special properties such as water resistance and filterability. Conventional lubricating oils for tractor are mainly required to improve extreme-pressure property to the gears and frictional property of wet-type clutch/wet-type brake (e.g., Patent Documents 1˜7).
In addition to the above properties, the lubricating oils used for tractors in cold region are particularly required to have sufficient cold flow for the hydraulic pump starting at low temperature. A cold flow for such as tractors of which BF viscosity at −40° C. is 20,000 mPa·s or less is known (e.g., Patent Document 5).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 6-200269    Patent Document 2: JP-A No. 6-240283    Patent Document 3: JP-A No. 7-109477    Patent Document 4: JP-A No. 9-165590    Patent Document 5: JP-A No. 9-165592    Patent Document 6: JP-A No. 2001-311090    Patent Document 7: JP-A No. 2004-059930